Abstract
Recent increases in glyphosate use in perennial crops of California, USA, are hypothesized to have led to an increase in selection and evolution of resistance to the herbicide in Conyza canadensis populations. To gain insight into the evolutionary origins and spread of resistance and to inform glyphosate resistance management strategies, we investigated the geographical distribution of glyphosate resistance in C. canadensis across and surrounding the Central Valley, its spatial relationship to groundwater protection areas (GWPA), and the genetic diversity and population structure and history using microsatellite markers. Frequencies of resistant individuals in 42 sampled populations were positively correlated with the size of GWPA within counties. Analyses of population genetic structure also supported spread of resistance in these areas. Bayesian clustering and approximate Bayesian computation (ABC) analyses revealed multiple independent origins of resistance within the Central Valley. Based on parameter estimation in the ABC analyses, resistant genotypes underwent expansion after glyphosate use began in agriculture, but many years before it was detected. Thus, diversity in weed control practices prior to herbicide regulation in GWPA probably kept resistance frequencies low. Regionally coordinated efforts to reduce seed dispersal and selection pressure are needed to manage glyphosate resistance in C. canadensis.
Highlights
Agricultural weeds regularly adapt to multiple selective pressures at the contemporary timescale, including climate and soil conditions (e.g., Begg et al 2012) as well as crop, soil, and weed management practices (e.g., Barrett 1983; Powles and Yu 2010; Owen et al 2011)
The recent spread of glyphosate resistance in C. canadensis of California mainly happened in the southern areas of the Central Valley (Fig. 3A; Hanson et al 2009)
Our results suggest that it was associated with increased selection by glyphosate due to recent regulatory restrictions on the use of herbicides other than glyphosate in groundwater protection areas (GWPA) that came into effect in 2004, as hypothesized by Shrestha et al (2007)
Summary
Agricultural weeds regularly adapt to multiple selective pressures at the contemporary timescale, including climate and soil conditions (e.g., Begg et al 2012) as well as crop, soil, and weed management practices (e.g., Barrett 1983; Powles and Yu 2010; Owen et al 2011). One of the best examples of weed adaptation to management practices is the evolution of resistance to herbicides (Maxwell et al 1990; Jasieniuk et al 1996; Powles and Yu 2010). Glyphosate has several favorable properties as a herbicide, including low mammalian toxicity, very low activity in the soil, and effectiveness on a diversity of species, which have made it a key weed management tool in modern agriculture (Baylis 2000; Duke and Powles 2008). Glyphosate use in agriculture has increased markedly in the last two decades due to the adoption of no-tillage and reduced tillage practices as well as the introduction of transgenic
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